Should a player be allowed to retire from Rugby League and then pursue a career in football?

No, not if he has a contract to fulfil in RL.
Unless there is agreement on both side's the contracts should be completed, after all, that is why people have them.
Remember, a contract is to protect both parties and hat would have happened had he been injured while playing for Cas, should they have "retired" him without paying up.

Sale appear to have tried to sign him "legitimately" and when Cas turned down their offer, they appear to have used some very dodgy ways to persuade him to switch codes.
Its not acceptable and if the same thing had happened in reverse, the media would be all over it.
Mind you, it's not likely to happen but, that's not the point.

Unsubstantiated rumour but I heard that Leeds (can't remember if it was Hetherington or Caddick) were very much behind Cas in this dispute and were prepared to assist financially if need be.

Are they not preoccupied financially with redeveloping Headlingley ATM?

So the RFL are behind Cas then.

He certainly didn't seem very depressed on the 'just tattoo of us' tv show (certainly not my choice of show. Filmed back around February.. Not sure someone who is depressed should be encouraged to get mystery tattoos they won't get to see until it's done...